PAS Staining for Fungal Detection
Periodic acid-Schiff (PAS) staining is indeed one of the most sensitive histopathological methods for detecting fungi and glycogen-rich structures in tissue specimens.
Why PAS is Highly Sensitive
PAS staining works by oxidizing polysaccharides (specifically 1,2-glycol groups) in fungal cell walls to create aldehyde binding sites that react with Schiff's reagent, producing a characteristic magenta-pink color 1. This makes it particularly effective for visualizing fungal elements that might otherwise be missed with routine hematoxylin and eosin (H&E) staining, where fungal structures can be misinterpreted as artifacts 1.
Guideline-Based Recommendations
Multiple infectious disease guidelines consistently recommend PAS as a primary fungal stain:
The German Society for Haematology and Oncology (AGIHO) guidelines explicitly state that bronchoscopic material or tissue biopsies should be examined with periodic acid-Schiff staining when fungal infection is suspected 1.
The 2016 IDSA Aspergillosis guidelines note that special stains on fixed tissue include both Gomori methenamine silver (GMS) and periodic acid-Schiff stains, and these should be performed simultaneously when fungal infection is suspected 2.
The 2018 ESCMID-ECMM-ERS guideline recommends that GMS and PAS can be applied to histological sections and should be conducted in all cases where invasive aspergillosis is considered 3.
PAS vs. GMS: The Evidence
While both PAS and GMS are effective, the evidence shows:
For onychomycosis specifically, research demonstrates that PAS and GMS stains are quantitatively similar in sensitivity 4. A 2010 study found PAS detected fungi in 22 of 30 cases versus 19 for GMS, though this difference was not statistically significant 4.
For general fungal detection, recent histopathology studies in dermatology found PAS positive in only 3% of palm/sole biopsies, with low concordance between clinical suspicion and actual fungal demonstration 5.
The 2014 British Association of Dermatologists guidelines note that histopathological analysis using PAS staining is more sensitive than direct microscopy or culture for onychomycosis 6.
Clinical Application Algorithm
When suspecting fungal infection:
Submit adequate tissue quantities for both histopathologic examination and culture simultaneously 2
Request both PAS and GMS stains upfront rather than sequentially - this maximizes detection sensitivity 1, 3
For optimal yield: Prepare slides from different levels in the paraffin block, as this increases sensitivity for both staining methods 4
Communicate clinical suspicion to pathology laboratory personnel to ensure appropriate stains are performed 2
Important Caveats
PAS stains polysaccharides non-specifically, meaning it will also highlight glycogen-rich structures, basement membranes, and mucin - not just fungi 7
Fixation and processing methods significantly affect staining quality - variations can produce subtle differences in tissue morphology and staining intensity 8
No histopathologic finding can definitively identify the specific fungal pathogen - culture correlation is essential for species identification 2
GMS may be superior for non-viable fungal structures, as it stains fungal cell walls regardless of viability 1
Bottom Line for Practice
Use PAS staining routinely when fungal infection is in the differential diagnosis, ideally alongside GMS staining for complementary detection 1, 3. The combination provides the highest diagnostic yield, with PAS being particularly sensitive for viable organisms and glycogen-containing structures, while GMS excels at detecting non-viable fungal elements 1, 7.